Since last September they had threatened to sue you in 90 days, they must be very busy if they wasted so much time before threatening to sue you again. What are they busy doing?
Did you check out the law firm as Robert suggested? Do they specialize in digital image law, or are they ambulance chasers? If the latter, they are likely trolls and will not sue. I would expect that since they are contacting you by email, they might be trolls because they can't be sure their email got through spam filters and the like.
Did you check out the picture registration personally? Robert gives good advice when it comes down to these kinds of details. Don't trust what they put it front of you. Make sure it's registered. And did you find where the writer sourced the picture? If the photographer put the photo on a creative commons site, they will likely not sue you.
Did you check out the law firm as Robert suggested? Do they specialize in digital image law, or are they ambulance chasers? If the latter, they are likely trolls and will not sue. I would expect that since they are contacting you by email, they might be trolls because they can't be sure their email got through spam filters and the like.
Did you check out the picture registration personally? Robert gives good advice when it comes down to these kinds of details. Don't trust what they put it front of you. Make sure it's registered. And did you find where the writer sourced the picture? If the photographer put the photo on a creative commons site, they will likely not sue you.